Band-gap circuits are well known devices that are used to provide a reference voltage that is relatively constant across a wide temperature range. Exemplary band-gap circuits are disclosed in U.S. Pat. No. 3,887,863 and U.S. Pat. No. 6,278,320. The disclosures of U.S. Pat. Nos. 3,887,863 and 6,278,320 are hereby incorporated by reference into the present disclosure as if fully set forth herein.
The theory of operation of band-gap reference circuits is well known in the art. Two different sized base-emitter diodes are biased with the same current level. Since the diodes are not the same size, the diodes operate in different current density. The differences in current density are used to generate a proportional-to-absolute temperature (PTAT) current. The PTAT current develops a voltage across a resistor, thereby creating a PTAT voltage. The PTAT voltage is proportional to absolute temperature and has a positive temperature coefficient. This voltage is then summed to a base-emitter junction voltage of a diode that has a negative temperature coefficient. The negative temperature coefficient and the positive temperature coefficient cancel each other out, so that the combined voltage across the resistor and the base-emitter junction is constant over temperature.
FIG. 1 illustrates conventional band-gap reference circuit 100 according to an exemplary embodiment of the prior art. Band-gap reference circuit 100 comprises capacitor 105, current sources 110 and 115, amplifiers 120 and 125, N-channel transistors 131-133, resistors 140 and 145, PNP bipolar junction transistors 151-153, amplifier 160, P-channel transistor 165, and resistor 170. PNP bipolarjunction transistors 151-153 are connected as diodes and are referred to hereafter as PNP diodes 151-153. According to an exemplary embodiment, PNP diode 151 has an area that is eight times larger than the area of PNP diode 152 (i.e., 8:1 ratio).
According to an exemplary embodiment of the present invention, controller 225 of cellular telephone 200 is capable of conserving power and prolonging the operating life of battery 230 by periodically shutting down blad-gap reference circuit 240, and many of the other electrical circuits in cellular telephone 200. If the turn-on time of band-gap reference circuit 240 is made extremely short (e.g., 2 microseconds) compared to the 100+ microseconds of conventional designs, cellular telephone 200 can be powered back up without any significant delay, thereby saving considerable power over time.
A temperature independent band-gap reference voltage, V(bg), is established by summing the voltage across a resistor (having a positive temperature coefficient) and the base-emitter voltage, V(be), of a pn junction of a pnp diode having negative temperature coefficient. Typically, the sizes of the pnp diodes are chosen with an 8:1 area ratios (the result of using common centroid matching geometry throughout the industry), as in the case of PNP diodes 151 and 152, so that the PNP diodes operate at unequal current densities.
Let:
1) PNP diode 151 be denoted as D1;
2) PNP diode 152 be denoted as D2; and
3) PNP diode 153 be denoted as D3.
From FIG. 1 it can be seen that:V(be)D2=V(be)D1+I1(Ri),  [Eqn. 1]where Ri is the resistance value of resistor 140.
The current, i, in a PNP diode is given by the equation:i=Is(ev(be)/VT),  [Eqn. 2]where i is proportional to area. Rearranging terms in Equation 2 gives:V(be)=VT[ln(i/IS)].  [Eqn. 3]
Substituting V(be) in Equation 3 into Equation 1 gives the expression:V(be)D2−V(be)D1=I1(Ri)=VT[ln(8iD1/iD1]  [Eqn. 4]where iD1 is the current in D1 (i.e., PNP diode 151) and iD2 is the current in D2 (i.e., PNP diode 152). Since iD1 and iD2 are equal, Equation 4 reduces to:I1(Ri)=VT(ln 8)  [Eqn. 5]
Thus, the current I1 in PNP diode 151 is:I1=VT(ln 8)/Ri.  [Eqn. 6]
It is noted that VT, the thermal voltage has a positive temperature coefficient, VT. =+26 mV, at room temperature. Thus, the current I1 is proportional to absolute temperature (PTAT).
The current I1 is mirrored by the current I3 in N-channel transistor 133. The current I3 may be used to establish a band-gap reference voltage, V(bg) for use in biasing, where:V(bg)=I3(k*Rr)+V(be)D3.  [Eqn. 7].By selecting a suitable multiplier, k, such that dV(bg)/dT=0, V(bg) becomes independent of temperature.
Furthermore, it is possible to generate a reference current, I4, that is proportional to V(bg). This is achieved by the feedback loop formed by amplifier 160, P-channel transistor 165 and resistor 170, which generate I4=V(bg)/Ro, where Ro is the resistance value of resistor 170.
As FIG. 1 shows, the band-gap circuit provides a temperature compensated reference voltage output for use by other circuits in a system. A temperature insensitive, high-tolerance band-gap reference circuit is an indispensable building block in modern chip level integrated circuits (ICs). Band-gap reference circuits are used for biasing analog circuits, as a reference level for data converters, to set trip points for comparators and sensors, and the like.
Some applications, such as data converters and low drop-out (LDO) voltage regulators, require low-noise characteristics and a high PSRR (power supply rejection ratio). Prior art devices may employ large value filter capacitor to improve noise and PSRR performance. However, this impacts system cost and board size and, worst of all, slows down turn-on time (i.e., the time it takes for the band-gap reference circuit to stabilize the output voltage after being turned on). For example, many cellular telephones conserve battery power by periodically turning off various circuit blocks. If the turn-on time is too long, it is not practical to shut off these circuits. This wastes power and impacts system performance. Since band-gap reference circuits are relatively slow to startup, it is necessary that a faster startup technique be incorporated to meet the current needs of cellular telephone and other similar power critical applications.
As mentioned, conventional band-gap reference circuit 100 consumes a relatively large amount of current (>100 microamperes) and is slow to start up (>100 microseconds). Additionally, many modern portable applications, such as cellular telephones and pagers, operate from a +1.2 power supply rail. The V(be) base-emitter voltage drops in band-gap reference circuit 100 leave very little voltage margin with which to operate.
Furthermore, the current (i) in a PNP diode, as defined in Equation 2, exhibits non-linear behavior at high temperature. This is a key element that leads to large variation of band-gap voltage over temperature. Reducing such a variation often requires the introduction of a suitable correction current. Prior art current correction devices require elaborate circuitry and trimming techniques to generate an appropriate non-linear correction current that mitigates the nonlinear behavior of the PNP diode current at high temperature. The result is a flatter band-gap voltage profile over temperature.
Therefore, there is a need in the art for an improved band-gap reference circuit that is capable of operating from a low voltage (e.g., +1.2 volts) power supply rail. More particularly, there is a band-gap reference circuit that uses a simple circuit to generate an appropriate non-linear correction current to correct the nonlinear behavior of the PNP diode current at high temperature.